1. Field of the Invention
The present invention relates to an electromagnetic focusing method for an electron-beam lithography system, and more particularly, to an electromagnetic focusing method, which can accurately focus the electron beam even when (a) the electric and magnetic fields are not in parallel with each other in a vacuum chamber of an electron-beam lithography system, (b) intensity of magnetic field is not uniform, and (c) there is Coulomb-interaction between electrons.
2. Description of the Related Art
In a semiconductor manufacturing process, a variety of lithography technologies have been used to process a surface of a wafer in a predetermined pattern. Among the technologies, an optical lithography has been widely used. However, there is a limitation in reducing a line width using the optical lithography. Accordingly, in recent years, a next generation lithography (NGL) technology that makes it possible to realize a semiconductor having an integrated circuit with a nano-scale line width has been developed. An electron-beam lithography, an ion-beam lithography, an extreme ultraviolet lithography, a proximity X-ray lithography are well known as such NGL technologies.
Among the technologies, the electron-beam lithography system is designed to use electron beam in order to develop an electron-resist deposited on a wafer in a predetermined pattern. The electron-beam lithography system has advantages of being formed in a simple structure and easily realizing a large-sized electron beam emitter, thereby quickly forming a complicated pattern at a time.
FIG. 1 schematically shows a conventional electron-beam lithography system.
As shown in FIG. 1, an electron-beam lithography system includes a vacuum chamber 10 defining a wafer processing space. The vacuum chamber 10 is vacuumized by a vacuum pump. An electron-beam emitter 20 for emitting electron beam is installed in the vacuum chamber 10. The wafer 30 is disposed facing the electron-beam emitter 20 and spaced away from the electron-beam emitter 20 by a predetermined distance. A mask 21 formed in a predetermined pattern is disposed on an emitting surface of the electron-beam emitter 20 so that electron beam can be emitted through a portion that is not covered by the mask 21. As a result, an electron resist deposited on the wafer 30 is patterned by the electron beam in a pattern identical to that formed on the emitting surface of the emitter 20. The wafer 30 is supported by a wafer holder 35 disposed in the vacuum chamber 10.
Upper and lower magnet assemblies 51 and 52 are respectively installed on top and bottom of the vacuum chamber 10 to form a magnetic field in the vacuum chamber 10. The upper magnet assembly 51 includes a core 51a formed of a ferromagnetic substance and a coil 51b wound around the cores 51a. The lower magnet assembly 52 includes a core 52a formed of the ferromagnetic substance and a coil 52b wound around the core 52a. Upper and lower pole-pieces 53 and 54 are installed penetrating the top and bottom of the vacuum chamber 10, respectively. The upper and lower pole-pieces 53 and 54 magnetically contact the cores 51a and 52a of the respective upper and lower magnet assemblies 51 and 52, respectively. The upper and lower pole-pieces 53 and 54 function to introduce magnetic flux generated by the upper and lower magnet assemblies 51 and 52 into the vacuum chamber 10.
Provided in the vacuum chamber 10 are electrode plates 41 and 42 forming an electric field between the emitter 20 and the wafer 30. The electrode plates 41 and 42 are disposed on respective rear surfaces of the emitter and wafer 20 and 30 and connected to a power supply. The electron beam emitted from the emitter 20 are directed onto the electron resist 31 formed on the wafer 30 by mutual reaction between the electric and magnetic fields respectively formed by the magnet assemblies 51 and 52 and the electrode plates 41 and 42.
In order to accurately direct the electron beam emitted from the emitter 20 onto the electron resist 31, components of the electron-beam lithography system should be precisely aligned to uniformly maintain the electric and magnetic fields. However, it is inevitable there is some alignment error during the manufacturing and assembly process of the system. However, when the alignment error of the components is too big, the electric field may not be in parallel with the magnetic field or the intensity of the magnetic field may be increased or decreased according to the distance between the emitter and the wafer. As a result, the electron beam may not be accurately landed on the target location. In addition, when the electron beam is emitted from the large-sized surface, the repulsive force between electrons according to Coulombs law may increase the beam diameter. In this case, an error caused by the assembly tolerance should be compensated by properly adjusting the intensity of the electromagnetic field. However, no accurate standard for adjusting the intensity of the electromagnetic field is provided, it is difficult and time-consuming to accurately focus the electron beam.